A definitive test for our hypothesis that calcium is a principal agent in the initiation of vision is the measurement of transient light-stimulated changes in calcium ion activity in the light-receiving visual cell, and for this purpose we investigated the properties of a calcium sensitive dye dichlorophosphonazo-III (DCP3) and methods of introducing it into cells. A major problem facing cell biologists is one of introducing into live cells for a variety of purposes large, electrically charged molecules such as DCP3 which are excluded by cell membrane. We have developed a method to overcome this part which now allows us to introduce DCP3 into visual cells. On the other hand, lipid soluble materials have no difficulty entering cells, but their insolubility and instability in physiological solutions hamper their introduction into cells in culture or perfusion. We have used phospholipid vesicles to deliver to live cells, water soluble, oxygen sensitive compounds like retinol. In this manner visual pigment can be restored in isolated retinas. The process of the restoration of visual pigment and its relation to the photoelectrical activity of the visual cell are now being studied.